Interim Considerations for Phased Implementation of COVID-19 Vaccination and Sub-Prioritization Among Recommended Populations


The Advisory Committee on Immunization Practices (ACIP) recommends that, when supplies of COVID-19 vaccine are limited, vaccination should be offered in a phased approach. Phase 1a includes healthcare personnel and long-term care facility residents. Phase 1b includes persons ≥75 years of age and frontline essential workers. Phase 1c includes persons 65-74 years of age, persons 16-64 years of age with high-risk medical conditions, and other essential workers.

ACIP defines frontline essential workers as the subset of essential workers likely at greatest risk for work-related exposure to SARS-CoV-2, the virus that causes COVID-19, because their work-related duties must be performed onsite and their duties involve being in close proximity (<6 feet) to the public or coworkers. Further information on the essential workforce can be found in the Cybersecurity Infrastructure and Security Agency’s (CISA) Guidance on the Essential Critical Infrastructure Workforceexternal icon.

These considerations apply in the context of limited vaccine supply during the first months of the national COVID-19 vaccination program and will be updated as needed based on changes in vaccine supply, COVID-19 epidemiology, or other factors. CDC has also published a companion guide to assist state, tribal, local, or territorial immunization programs and other immunization partners in planning for vaccination of these populations.

Transitioning Between Vaccination Phases

Considerations for transitioning between phases are needed to ensure expeditious transition from one phase of COVID-19 vaccine allocation to the next (i.e., from Phase 1a to 1b or from Phase 1b to 1c) as vaccine supply increases and exceeds demand within specific populations or geographic locations in a given phase, or when low demand puts vaccine doses at risk for going unused. It is not necessary to vaccinate all individuals in one phase before initiating the next phase; phases may overlap.

CDC offers the following considerations for transitioning from one phase to the next, recognizing that these considerations will likely need to be adapted to the local, state, or territorial context and may change over time. These considerations may also be applied to transitions among groups sub-prioritized within a phase, in conjunction with factors specific to jurisdictional needs.

Currently two COVID-19 vaccines have been authorized under an Emergency Use Authorization issued by the Food and Drug Administration; both vaccines consist of a 2-dose series. These suggested considerations refer to planning for allocation of first doses of a COVID-19 vaccine series. Second doses need not factor into decision-making regarding the transition between phases because, as the first doses are allocated, the corresponding second dose allocations are being held in reserve at the federal level to ensure an adequate number of doses are available for completion of the series in all those who receive a first dose.

Decisions regarding transition from one phase to the next should be made at the local, state, or territorial level, and may be based on factors such as demographic and workforce characteristics, COVID-19 epidemiology within the jurisdiction, and vaccine supply and demand within the jurisdiction.

Once COVID-19 vaccination has been expanded to additional phases, jurisdictions should continue to offer and promote vaccination to all persons in earlier phases who have not yet been vaccinated. If vaccination sites within a jurisdiction are moving independently through the phases and are more than one phase apart (e.g., some sites are ready to move to Phase 1c while others are still in Phase 1a), jurisdictions should assess vaccine supply, demand, and equitability of vaccine distribution, and consider re-distribution of vaccine or ways to expand vaccination access, demand, and capacity, and address vaccine hesitancy within the jurisdiction. For example, jurisdictions may need to reassess and increase efforts to support people in making their vaccination decisions.

Considerations for Transitioning Between Phases

Decisions about transitioning to subsequent phases should depend on supply, demand, equitable vaccine distribution, and local, state, or territorial context. These suggested considerations are meant to help inform assessments of current phase and decisions on when to transition to the next phase.

Jurisdictions may consider expanding vaccine availability to priority groups in the next phase in several situations, including, for example:

  • When demand in the current phase appears to have been met (e.g., appointments for vaccination are < 80% filled for several days)
  • When supply of authorized vaccine increases substantially (e.g., more vaccine doses are available than are necessary to complete vaccination of persons in the current phase)
  • When most people in the current phase are vaccinated (e.g., when approximately 60-70% of the target population in a phase has been vaccinated)
    • In some populations, it may be difficult to reach these thresholds. In situations where jurisdictions have actively worked to increase both access and demand among populations in a specific phase (e.g., by identifying and addressing social and structural barriers to vaccination to the extent possible) but vaccine supply still exceeds demand, jurisdictions may consider transitioning to the next phase in order to ensure that vaccine doses are used and to maximize vaccine uptake.
  • When vaccine supply within a certain location is in danger of going unused unless vaccination is expanded to persons in the next phase

Sub-prioritization of Frontline and Other Essential Workers

Frontline essential workers and other essential workers have been recommended by CDC’s Advisory Committee on Immunization Practices (ACIP) to receive vaccination in Phases 1b and 1c, respectively.

Frontline essential workers recommended by ACIP to receive vaccination in Phase 1b are: firefighters, police officers, corrections officers, food and agricultural workers, U.S. Postal Service workers, manufacturing workers, grocery store workers, public transit workers, and those who work in the education sector (teachers and support staff members) as well as daycare workers. Other essential workers recommended by ACIP to receive vaccination in Phase 1c include people who work in: transportation and logistics, water and wastewater, food service, housing construction, finance, information technology, communications, energy, legal services, media, public safety, and public health.

In settings where the vaccine supply is insufficient to vaccinate all workers in a given phase (e.g., frontline essential workers in Phase 1b, other essential workers in Phase 1c), state, local, and territorial jurisdictions will need to decide how to sub-prioritize groups of people for COVID-19 vaccination. As jurisdictions make these complicated decisions, they will need to keep in mind that the intent of the recommendations is to ensure equitable access to vaccination among people at highest risk for severe disease and those who perform essential services and functions. Sub-prioritization among groups of essential workers may differ by jurisdiction, given the potential variability in the composition of non-healthcare essential workers required to ensure continuity of functions that are critical to public health and safety, security, and the economy at jurisdictional and national levels.

Considerations for Sub-prioritization

Within a given phase, jurisdictions may consider sub-prioritization of the following groups of workers:

  • Groups of workers that are the most critical to maintaining core societal functions
  • Groups of workers with unavoidable higher risk of exposure because of their inability to perform work duties remotely and who work without adequate protection while in close proximity to coworkers or members of the public (e.g., persons in meatpacking plants who might be working in close quarters or personal care assistants who are providing care for individuals in group homes)
    • Considerations may be informed by the length of time workers are exposed to each other and the public and the number of contacts they have during a typical workday.
  • Groups of workers in sites where high rates of transmission and outbreaks can occur regularly (e.g., correctional and detention facility workers)
  • Groups of workers that have been disproportionately affected by COVID-19 (e.g., persons in groups impacted by health disparities and inequities such as migrant farm workers)

Some individuals within an essential worker category are at higher risk of severe illness or may be more likely to transmit disease to others. Following prioritization of frontline essential worker groups or other essential worker groups (depending on the jurisdiction’s current phase of allocation), jurisdictions may recommend that immunization providers consider prioritizing vaccination of:

  • People who are at increased risk for severe illness from the virus that causes COVID-19 based on age or self-identified underlying medical conditions
  • Persons/groups at increased likelihood of transmitting infection to others outside of work or live in settings where high level of transmission and outbreaks have occurred (e.g., workers who live in temporary group housing or workers who live in multigenerational homes)
  • People who do not have a history of documented acute SARS-CoV-2 infection in the preceding 90 days.
    • Workers with a history of SARS-CoV-2 infections within the preceding 90 days may choose to delay vaccination until near the end of the 90-day period in order to facilitate vaccination of those workers who remain susceptible to infection, as current evidence suggests reinfection is uncommon during this period after infection.
    • Of note, previous SARS-CoV-2 infection, whether symptomatic or asymptomatic, is not considered a contraindication to vaccination, and serologic testing for SARS-CoV-2 antibodies is not recommended prior to vaccination.

Considerations for Other Populations in Phase 1b and 1c

Sub-prioritization may also be necessary among groups included in Phase 1b and 1c based on factors other than essential worker classifications, including age and underlying medical conditions.

Underlying medical conditions:

CDC has identified a list of medical conditions that increase an individual’s risk of severe COVID-19. Persons with underlying medical conditions not identified as a high-risk condition may also be at increased risk of severe disease and should consult with a healthcare provider about personal risk factors. As COVID-19 vaccines become more widely available in providers’ offices and pharmacies, healthcare providers may use clinical judgement to determine an individual patient’s priority for vaccination.


Currently, adults who are 75 years of age and older are included in Phase 1b, and adults 65-74 years of age are included in Phase 1c.  When vaccine supplies are limited, sub-prioritization of adults in both groups may be needed, such as prioritization of those with underlying medical conditions over older adults without any of those conditions. When supply is sufficient, state, local, or territorial jurisdictions may consider moving quickly from Phase 1b to Phase 1c, to allow for expanded access of COVID-19 vaccines to all adults 65 years of age and older.

Communities that have been disproportionately affected by COVID-19 and have a younger population distribution, such as American Indian and Alaska Native communities, may choose to adopt a different lower age limit for vaccination prioritization appropriate to state, local, or territorial epidemiology.

Congregate living settings:

Increased rates of transmission have been observed in congregate living settings. Based on local, state, or territorial epidemiology and implementation considerations, jurisdictions may choose to vaccinate persons who reside at congregate living facilities (e.g., correctional or detention facilities, homeless shelters, group homes, or employer provided shared housing units) at the same time as the frontline staff, because of their shared increased risk of disease.

Page last reviewed: December 23, 2020